In a section of such a propeller pump, a so-called diffuser having guide vanes is found, which is a section of the propeller pump wherein deflection of the flow of the liquid stream as well as pressure recovery occurs after the liquid has left the propeller of the propeller pump. More precisely, the function of the diffuser and guide vanes is to deflect/reroute the liquid stream that leaves the propeller of the propeller pump in the rotational direction as well as in the radial-axial direction, with the purpose of obtaining an output liquid stream from the diffuser and primarily parallel to the rotation axis of the propeller pump. Construction-wise, this means, among other things, that the pump core has a convex shape downstream the propeller in order to deflect the liquid stream partly directed radially outward that leaves the propeller into an axial liquid stream.
When the diameter of the convex surface no longer increases, but changes to levelling out or slightly decreasing, a negative pressure gradient is provided that is directed upstream parallel to the envelope surface of the pump core and that is adjacent to the envelope surface of the pump core. This so-called negative axial pressure gradient increases when the radius of curvature of the convex surface begins to increase, i.e., begins to level out, as viewed in the axial direction, which implies that a boundary layer closest to the envelope surface of the pump core also increases in the direction downstream. The boundary layer exhibits a considerably reduced speed and eventually a rearwardly directed speed, or reverse flow/stream. In order to avoid that the negative axial pressure gradient becomes so large that reverse flow arises, it is known to make the radius of curvature of the convex surface sufficiently large. However, this implies the disadvantage that the axial length of the propeller pump increases, which is most pronounced for large propeller pumps having a diameter of approx. 1-2 m and an axial length of approx. 3-4 m.
Furthermore, the area wherein the suction side of the guide vane meets the envelope surface of the pump core is extra susceptible to separation, i.e., emergence of reverse flow. This depends foremost on the boundary layer along the envelope surface of the pump core being added to a such a boundary layer that is present on the suction side of the guide vane in the area where the radius of curvature of the convex suction side of the guide vane begins to increase, as viewed in the axial direction, which in summary leads to further increased risk of reverse flow and thereby large losses.